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Author Topic: lower EC or ETTR?  (Read 22194 times)
Graystar
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« Reply #80 on: August 22, 2012, 10:03:20 PM »
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According to my own tests with the 800e, if one is using a uniform target which occupies the whole field of view, the reading are the same regardless of the metering mode. This is also what is stated on the Rawdigger site:

"the grey target (a grey patch can also be used, as well as plain white paper, without optical whiteners). Turn off all exposure adjustments and shoot. If the target takes up the entire field of view of the shot, then the setting of the exposure meter does not matter; all settings (spot, matrix, center-weighted) will result in the same exposure. "

Technically, that's correct.  However, Matrix meters the entire frame, which includes falloff, and that can cause it to increase exposure over a spot meter reading.  And while the final image may not have much falloff due to the aperture use, metering always occurs wide open...so falloff is always an influence.  Obviously, the lens used is also a factor.  In any case, I've seen spot metering variations on my gray card from RGB 90 to RGB 110 in my D90, depending on the lens and conditions.  So the RGB 108 of your Exp1.NEF file is within the ball park.

My statement that started all this was, "If there's any white in the scene, then you can't ETTR."

Which I further clarified with, "Yes, ETTR is a sound concept.  Just have to realize that on scenes with white objects and bright highlights, standard exposure is pretty much the same as an exposure arrived at through ETTR."

Clearly, this applied to the D800.
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Graystar
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« Reply #81 on: August 22, 2012, 10:04:56 PM »
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Not for nothing but you all realize this thread is in the Beginners' Questions forum? Ya all might want to keep that in mind...perhaps a new thread not here would be in order. Pretty sure many/most readers of the thread have run away and hidden already.
You're right, but I think it's practically over now.
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bjanes
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« Reply #82 on: August 23, 2012, 09:38:02 AM »
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Technically, that's correct.  However, Matrix meters the entire frame, which includes falloff, and that can cause it to increase exposure over a spot meter reading.  And while the final image may not have much falloff due to the aperture use, metering always occurs wide open...so falloff is always an influence.  Obviously, the lens used is also a factor.  In any case, I've seen spot metering variations on my gray card from RGB 90 to RGB 110 in my D90, depending on the lens and conditions.  So the RGB 108 of your Exp1.NEF file is within the ball park.

You raise a good point that I had not considered. In my previous test, I minimized falloff by using f/8 with a 105 mm lens. I repeated the tests with the Zeiss Distagon 21 mm f/2.8, which is an excellent lens but is known to have considerable falloff. Rather than upload the large raw files, I chose to merely show the results. However, the matrix metering results in lower saturation in my tests.







These differences are not large, but could affect critical ETTR and should be accounted for in critical work. Jeff Schewe may object to these arcane matters in a thread for beginners, but good discussions often veer off in an interesting fashion, and help beginners progress to a more advanced stage.

Regards,

Bill
« Last Edit: August 23, 2012, 09:52:35 AM by bjanes » Logged
Graystar
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« Reply #83 on: August 23, 2012, 01:17:49 PM »
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You raise a good point that I had not considered. In my previous test, I minimized falloff by using f/8 with a 105 mm lens. I repeated the tests with the Zeiss Distagon 21 mm f/2.8, which is an excellent lens but is known to have considerable falloff. Rather than upload the large raw files, I chose to merely show the results. However, the matrix metering results in lower saturation in my tests.

That's odd.

I have a different method of demonstrating the falloff effect.  I set the camera on a tripod to photograph a gray card, and then I take 4 images.  The first two, Spot and Matrix, are with a light source very close to the card so that the card exhibits falloff.  The second two, also Spot then Matrix, are with the light source moved further away to minimize falloff.  The result is that my first two images have different exposures, but the next two images don't...even though the only change to the camera, between the images of each pair, was the metering mode.  So that demonstrates that the falloff is affecting Matrix metering.

However, in my shots with the light source close to the gray card, the Matrix image is clearly brighter...which, to me, would be the expected result of a metering mode that is averaging dark areas with bright areas.  So I don't know why you got the opposite result.  I've put the NEF files on my FTP...

ftp://graystar.tftpd.net (this address has to be copied and pasted into the address bar, as the forum software is adding an "http://" in front of it.)

DSC_9310 - light source close, Spot (f/8, 1/640s)
DSC_9311 - light source close, Matrix (f/8, 1/400s)
DSC_9312 - light source far, Spot (f/8, 1/20s)
DSC_9313 - light source far, matrix (f/8, 1/20s)

EDIT: The only time you get standard exposure from Matrix is when Matrix fails to match a scene in its database.  Just wanted to note that with the D800's expanded scene database, it's possible that Matrix found a match between your image and one of its scenes, and applied the exposure compensation related to that scene (this is what makes Matrix so unpredictable.)
« Last Edit: August 23, 2012, 02:14:41 PM by Graystar » Logged
bjanes
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« Reply #84 on: August 23, 2012, 03:00:14 PM »
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That's odd.

I have a different method of demonstrating the falloff effect.  I set the camera on a tripod to photograph a gray card, and then I take 4 images.  The first two, Spot and Matrix, are with a light source very close to the card so that the card exhibits falloff.  The second two, also Spot then Matrix, are with the light source moved further away to minimize falloff.  The result is that my first two images have different exposures, but the next two images don't...even though the only change to the camera, between the images of each pair, was the metering mode.  So that demonstrates that the falloff is affecting Matrix metering.

However, in my shots with the light source close to the gray card, the Matrix image is clearly brighter...which, to me, would be the expected result of a metering mode that is averaging dark areas with bright areas.  So I don't know why you got the opposite result.  I've put the NEF files on my FTP...

ftp://graystar.tftpd.net (this address has to be copied and pasted into the address bar, as the forum software is adding an "http://" in front of it.)

DSC_9310 - light source close, Spot (f/8, 1/640s)
DSC_9311 - light source close, Matrix (f/8, 1/400s)
DSC_9312 - light source far, Spot (f/8, 1/20s)
DSC_9313 - light source far, matrix (f/8, 1/20s)

EDIT: The only time you get standard exposure from Matrix is when Matrix fails to match a scene in its database.  Just wanted to note that with the D800's expanded scene database, it's possible that Matrix found a match between your image and one of its scenes, and applied the exposure compensation related to that scene (this is what makes Matrix so unpredictable.)

My method was to illuminate a white screen with 6 Solux lamps equipped with diffusers so that the illumination was uniform. The falloff is entirely due to the lens. With your setup, falloff can be related to both scene illumination as well as lens vignetting. However, the effect on the metering system and the sensor should be the same. Your explanation is reasonable. The take home point is that if one wishes to apply compensation to the metered value (ETTR for example) one should not use matrix, since a compensation has already been applied and the manual compensation may be in the same or opposite direction.

Regards,

Bill
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